can I manually plot different values (altitudes, temperature, cost index etc) to create my own performance charts when I don't have access to the data that the FMS uses to calculate the values? I know I'm reinventing the wheel, but if I can plot this as I'm flying, it will help me to understand the relationship between these values, ie altitude, temperature, weight, etc
Boeing publishes a manual which you can download here:
Jet Transport Performance Methods
This gives the principles for performance calculations and also the reference wing area for Boeing types, from which you can calculate coeficient of lift. However in order to replicate the FMC I think you would also need the airframe specific drag polars and engine fuel flow data which are not given in detail.
If you look at page 32-20 figure 32-21 there is an example of how to determine best endurance (holding) speed for a B757 at FL350. We previously discussed the theory about speeds for range and endurance of "jet" aircraft which is usually taught to ATPL students. According to this theory, best endurance occurs at the speed for minimum drag. The theory implies constant TSFC, i.e the blue fuel flow lines in figure 32-21 would be horizontal . However in this Boeing example the fuel flow lines slope downwards (TSFC decreases with speed) such that the speed for best endurance is about 23 knots TAS (13 knots EAS) slower than minimum drag speed.
I think this is a significant difference. In this example best endurance speed may perhaps be closer to Vmp than Vmd. Should we be teaching student pilots theories about range and endurance of "jet" aircraft which are quite innacurate for modern turbofan transport types?